Julian Day Converter — Gregorian Date to Julian Day
The Julian Day Number is the continuous day-count every chart on this site is actually computed from internally, since it sidesteps calendar irregularities that trip up date arithmetic. This tool converts freely between a normal Gregorian calendar date and its real Julian Day Number in either direction, down to fractional-day precision.
Julian Day numbers are the standard astronomers and astrologers actually compute with internally precisely because a Gregorian date has leap years, varying month lengths, and calendar-reform gaps that make direct date arithmetic unreliable — converting to a single continuous day-count sidesteps all of that.
Astronomers have used a continuous Julian Day count since the 16th century precisely to avoid this kind of calendar friction, and it remains the standard reference frame for orbital-mechanics software today — which is exactly why every chart on this site is computed against it internally before being translated back into a normal calendar date for display.
Frequently Asked Questions
What is Julian Day Number 0?
Noon UTC on January 1, 4713 BCE in the proleptic Julian calendar — a fixed epoch astronomers agreed on so every later date could be expressed as one continuous day count.
Why does the Julian Day start at noon instead of midnight?
It's a historical convention from astronomers, who wanted the date to stay the same across a single night's observing session rather than flipping over at local midnight partway through.
What's the difference between Julian Day and Modified Julian Day?
Modified Julian Day (MJD) is JD minus 2,400,000.5 — a smaller, more convenient number introduced in the 1950s that also starts at midnight instead of noon.
Does this converter use the Julian or Gregorian calendar?
Proleptic Gregorian throughout, for both directions of the conversion — the same convention every other calculation on this site uses internally.
Why does every chart on this site need a Julian Day Number first?
Because the underlying orbital-mechanics formulas for planetary positions, house cusps, and returns are all expressed as a function of elapsed days, not calendar dates.